Harper formed Yellowcard in 1997 along with Longineu W. Parsons III, Ben Dobson, Todd Clary, Warren Cooke, and Sean Mackin after meeting at Douglas Anderson School of the Arts. The band released their first album, Midget Tossing, in 1997. They then released their second album, Where We Stand in 1999. After Dobson left the band Harper asked friend Ryan Key to be the band's new lead singer. The band then released the Still Standing EP in early 2000. Shortly after the release of their EP Todd Clary left the band, Key then filled in as the rhythm guitarist. The band then moved to Camarillo, CA, after being signed to Lobster Records. They then begin working on their next album.

The group released their third album, One for the Kids in 2001 and followed up with The Underdog EP[1] in 2002. After the release of The Underdog EP, Warren Cooke left the band due to personal reasons. The band later asked close friend Peter Mosely from Inspection 12, to become the band's new bass guitarist. After Yellowcard released The Underdog EP the band signed with Capitol Records, who were looking to sign pop punk bands at the time. Yellowcard then began the recording of their major-label debut album Ocean Avenue in late 2002.

Mosely left during recording of the album due to personal problems. The band then asked Alex Lewis to join as the bass guitarist. Yellowcard released their album Ocean Avenue on July 22, 2003 reached No. 23 on U.S. Billboard 200, the album was a commercial success in the United States with the hit singles Way Away, Ocean Avenue, and Only One.

In early 2004, Lewis left the band after Mosely rejoined as the band's bassist. In 2005, Harper went with the other band members to Los Angeles while Key and Mosely moved to New York City to create new material for their next album. During the following months tensions arose between the band members. In an interview with MTV News he said that he was finished with writing the guitar tracks for the album and that he was leaving the band. He was replaced by Ryan Mendez from the band Staring Back.

After leaving Yellowcard, Harper joined Amber Pacific as their rhythm guitarist, replacing Justin Westcott. Harper left the band before recording their second studio album, Truth in Sincerity in October 2006.[2]

In 2008 Harper joined the band as the new lead guitarist alongside Steven Neufeld, Justin Cabrera, and T.J Arriaga. HeyMike! reconstructed its original trio in 2009 when Josh McDonald came back to continue his bass playing duties. Harper remained with the band and the quartet toured in the United States and Japan throughout 2009.[1] In 2010, HeyMike! finished recording their third record and second full length album titled Pop and Circumstance which is Harper's first recorded appearance since Yellowcard's Lights and Sounds. HeyMike! also toured in Europe and played a limited engagement of shows in the United States. "Pop and Circumstance was released on April 19, 2011.

In June 2014, Harper formed a new band with former Yellowcard drummer and good friend Longineu W. Parsons III, Chris Castillo (lead vocals and rhythm guitar) and Steven Neufeld (bass guitar) called This Legend.[3] The band played several shows across the U.S.